This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A158914 #6 Jun 14 2025 13:34:04 %S A158914 7,47,157,3863 %N A158914 Primes p such that there is a composite c with sigma_2(p)=sigma_2(c). %C A158914 No other terms less than 10^8. %C A158914 The corresponding composite numbers are 6, 40, 136, and 3352. %C A158914 Is this sequence finite? %C A158914 See A158913 for the sequence for sigma_1. %C A158914 Terms 47, 157, 3863 are x values of solutions to Pell-Fermat equation x^2 - 85*y^2 = 84, where y is also prime and c = 8*y. For any other solution (x,y) formed by primes, x is a term. - _Max Alekseyev_, Jun 14 2025 %t A158914 tp=DivisorSigma[2,Select[Range[4000],PrimeQ]]; tc=DivisorSigma[2,Select[Range[4000],!PrimeQ[ # ]&]]; Sqrt[Intersection[tp,tc]-1] %K A158914 more,nonn %O A158914 1,1 %A A158914 _T. D. Noe_, Mar 30 2009